This invention relates to a portable GPS receiver adapted to receive signals from GPS (Global Positioning System) satellites and measure locations and speeds of the receiver. More particularly, the invention relates to a GPS receiver which can be held by or worn on a human arm in order to measure locations, moving speeds and moving distances during running, walking, or other movement of the human body.
Conventionally, the GPS system has 24 GPS satellites revolving at a rate of 12 hours per turn on six orbits at an inclination angle of 55 degrees above approximately 20,200 Km around the earth. The navigation data required for positioning is transmitted from three to four or more satellites, and received by a receiver installed on the earth so that a mobile body having the receiver mounted therein may have calculated its position such as location, moving speed, etc.
Although the transmission wave by the GPS involves two kinds, i.e., L1 with a frequency of 1.57542 GHz and L2 with a frequency of 1.22760 GHz, ordinary positioning utilizes only L1. L1 is subjected to PSK modulation by a pseudo noise code (a synthetic wave of a C/A code for satellite identification and navigation data such as satellite orbit information, time information, etc.) and spread spectrum, to be transmitted.
FIG. 15 shows a block diagram of a GPS receiver for receiving radio waves as stated above. In the figure, 1501 is an antenna for receiving radio waves transmitted from the GPS satellites, 1502 is a L-band amplifying circuit for amplifying a received L-band signal, 1503 is a down-converter for performing signal conversion as described below, 1504 is a voltage comparator for digitally converting a signal supplied from the down converter 1503, 1505 is a message decoding circuit for obtaining carrier-wave phase information corresponding in pseudo distance to the navigation data, and 1506 is a C/A code generating circuit for generating C/A codes, 1507 is a position calculating section for calculating position data. These constituent elements are systematically connected therebetween, constituting the GPS receiver.
The GPS receiver structured as described above performs signal reception, as explained hereinbelow. The 1.57542 GHz signal received by the antenna 1501 is amplified by the L-band amplifying circuit 1502. This amplified signal is converted by the down-converter 1503 into an first IF (intermediate frequency) signal of several tens of MHz-200 MHz, and then into a second IF signal of approximately 2 MHz-5 MHz. This second IF signal is inputted to the voltage comparator 1504 and digital-converted with a clock several times the IF signal. Spread spectrum data is obtained as an output from the voltage comparator.
In the massage decoding circuit 1505, the digital signal outputted by the voltage comparator 1504 is subjected to reverse spread spectrum with a C/A code, i.e., the same pseudo noise code as that of the satellite, which is generated by the C/A code generating circuit 1506. Thus, carrier-wave phase information is obtained that is corresponding in pseudo distance to the navigation data. This operation is performed on a plurality of satellites. The position calculating section 1507 determines position data from the navigation data, usually, of four satellites. The position data determined by the position calculating section 1507 is supplied to the CPU for controlling operations of the entire portable apparatus, or otherwise to the outside as a digital signal.
Recently, GPS receivers as described above have been utilized for a vehicular navigation apparatus. Meanwhile, GPS receivers are also made very small and utilized as a portable apparatus for the purpose of determining a direction of a human body or a moving distance during walking, as disclosed by "Signal Receiver" in Japanese Laying-open Patent Publication No. H6-18156.
Where the conventional GPS receiver as described above is utilized to measure a moving speed or distance of a human body, if the GPS receiver is for example of a vehicular mounting type, the use of a self-navigating means such as map-matching enables a navigating operation to continue even where positioning is difficult to effect such as in tunnels or building valleys. However, where such a GPS receiver is utilized as a compact receiver for example in a portable form, it becomes difficult to incorporate CD-ROM map information therein due to smallness in size.
Also, where considering a vehicle, the moving distance or speed can be obtained from instruments installed on the vehicle. However, where the receive is of an on-arm type, the moving distance is determined from the GPS satellite. Consequently, if the satellite information becomes impossible to receive, there is a fear that the distance measurement is also impossible to carry out. Further, a human body will frequently vary in direction of movement. To accurately determine a moving distance requires continuously performing the operation of positioning. This, however, results in a problem in that the GPS receiver has an increased power consumption.